The power handling capability of a single capacitive element can be limited by power dissipation, voltage breakdown or, especially in the case of varactors, excessive capacitance distortion due to applied RF voltage.
In many resonators, it is desirable to combine multiple capacitive elements into a single Thevenin equivalent capacitor with increased power handling capability. It should be noted that capacitive elements can mean discrete capacitors, voltage variable capacitors, etched capacitors on a substrate, or combinations thereof. In high frequency resonators, it is difficult to connect several capacitors to a single discrete inductor. A popular solution is to connect the several capacitors to a distributed inductance.
One logical configuration for such a distributed inductor is a shorted coaxial line, as illustrated in FIG. 1. The end plate 10 short circuits the outer conductor 12 and inner conductor 14 at one end. Capacitors 16 couple the outer conductor and inner conductor at the other end.
The shorted coaxial resonator is advantageous in that the separation of the conductors can be as large as necessary to contain a desired number of radially connected capacitors without affecting the inductance of the resonator. The inductance of the shorted coaxial line is expressed by the following equation: EQU L=(Z.mu./2.pi.)*1n(b/a) (1)
where Z is the length of the line, .mu. is the magnetic permeability of free space, b is the radius of the line's outer conductor, and a is the radius of the line's inner conductor. The inductance is a function of the ratio of the radii of the outer and inner conductors and is not dependent on the absolute diameter of the shorted coaxial line.
All distributed resonators exhibit resonance at a number of different frequencies. Establishing the desired resonance mode to be the dominant mode is critical in applications, such as oscillators, that are susceptible to operation at several frequencies. The desired resonance mode is a transverse magnetic (TM) wave in the axial direction of the shorted coaxial line, as illustrated in FIGS. 2a and 2b. The magnetic field lines are perpendicular (transverse) to the direction of wave propagation. The electric field lines are radially symmetric and equal in magnitude and sign in any cross-sectional plane of the resonator. Since the electric field lines are symmetric, each radial capacitor leg will receive an equal share of the resonator power.
While this resonator is advantageous in certain respects, it is disadvantageous in others. Since the resonator is, by nature, a distributed circuit element, a distributed coupling technique is typically employed. Such techniques generally involve electromagnetic coupling to the resonator, such as by a coupling loop (as shown in U.S. Pat. No. 3,735,286), electrode or probe that causes an electromagnetic field to propagate into the resonant structure. Such coupling techniques are disadvantageous in certain applications that require a high degree of coupling to the Thevenin equivalent of the resonator.
A second disadvantage in coupling to the short-circuited coaxial resonator is a difficulty in establishing a desired resonance mode. General coupling techniques can excite several different modes of resonance. One disadvantageous mode is the transverse electric (TE) mode, as illustrated in FIGS. 3a and 3b. The electric field is perpendicular (transverse) to the direction of wave propagation, and in any cross-sectional plane, the electric field does not have a radial distribution. This wave causes unequal power division of the resonator power into the capacitors.
In accordance with the preferred embodiment of the present invention, these drawbacks are overcome by providing a coupling port to a multiple capacitor, short circuited coax resonator. This port is defined by adding a second short circuited coax line across the end of the first. The outer conductors of the lines are interconnected. The inner conductor of the second line can be a wire, cylindrical element or reactive element such as a coil. The outer conductor of the second line can be cylindrical or a finite approximation, such as a hexagonal can, for ease of manufacturing. The inner conductors of the two lines are serially coupled and define, either along their length or at their ends, a coupling gap across which discrete circuitry can be connected. In a preferred form of the invention, the discrete circuitry is positioned in a region within the periphery of one of the inner conductors in order to provide an electromagnetic shield for the circuitry. By maintaining the radial symmetry of the resonator and coupler, the dominant resonance mode is a TM wave. The coupling port presents to the discrete circuitry a Thevenin equivalent tuned circuit composed of the sum of the capacitance of the symmetric legs in parallel with the inductance of the short circuited coaxial line.
The foregoing and additional features and advantages of the present invention will be more readily apparent from the following detailed description thereof, which proceeds with reference to the accompanying drawings.